This invention relates to capacitor bank protection equipment such as series capacitor installations in high voltage alternating current transmission lines.
It is the practice in applications of capacitors, such as for power factor correction in transmission lines as well as other applications, for capacitors to be arranged in multi-series groups. That is, the bank comprises as many individual capacitor units in parallel with each other as is necessary to satisfy requirements for current carrying capacity and as many groups of capacitor units connected in series as is necessary to achieve the required amount of ohms or reactive capacitive impedance. Protective by-pass equipment is usually provided so that the capacitors can be protected against transient conditions that may stress them beyond their inherent design capability. A principal form of protection equipment is one that has across each group of parallel connected capacitor units a spark gap set at a protection level sufficient to ensure the capacitors are not susceptible to failure due to transient line conditions. However, in high voltage equipment, such spark gaps and their associated inductive reactors are expensive items that have a substantial impact on the total cost of the capacitor installation. The cost is further aggravated by the fact that the spark gaps generally available for use in such high voltage installations are not inherently selfextinguishing. That is, it is necessary to provide some means, such as air-blast equipment, for extinguishing the arc after a transient occurs that causes gap firing.
In the art of surge arresters generally, in addition to spark gaps, there are various known solid state suppressor devices used either by themselves or in combination with spark gaps to provide a desired function. Some of these, such as silicon carbide devices, are also not inherently self-extinguishing and current conduction through them will persist after the termination of a transient that initiates the conduction. Others, such as semiconductor zener diodes, employed in a back-to-back configuration, can achieve a high degree of reliable operation without ancillary equipment to terminate conduction after firing but are generally limited as a practical matter to relatively low voltage applications. Other known devices include metal oxide voltage limiters, such as those comprised principally of zinc oxide, which are non-linear resistors, as are silicon carbide devices, but which have a more non-linear characteristic and the ability to terminate conduction upon reduction of voltage level. Such devices are of interest for applications in capacitor protective equipment but in many such potential applications the commercially available devices today are not readily capable of sustaining the degree of energy absorption required and the duty cycle that the devices might be subject to.
Generally, what the present invention achieves is a protection apparatus for capacitor banks in a multiseries group arrangement in which available components are used in a novel combination to achieve both a high degree of reliable protection with minimized costs in terms of the number and nature and quality of required elements.
To set the more particular context in which the present invention arises, it is the case that in multiseries group capacitor banks, each unit has a fuse electrically in series and in close proximity to it. Sometimes such fuses are in fact contained within the capacitor unit itself; otherwise, they are normally on the immediate exterior of the capacitor unit. Current limiting fuses are one type of fuse that has an inherent capability of prevention of build-up of current during its clearing operation. Other fuses that are not current limiting have no such capability and there may be a sharp rise in current before the fuse clears in a brief time after the initiation of the condition causing its operation. Current limiting fuses are therefore preferable in most applications although more costly. The present invention is particularly advantageous in applications of fused capacitor units of any type although the problem addressed is more severe with non-current limiting fuses.
The fuse is intended to remove from service a capacitor unit that is simply not performing at its design level. Out of a bank of dozens of units, there is, of course, a statistical probability that, over time, some small percentage of units will fail and the overall operation of the bank, which is designed with some margin, can be maintained if a defective unit is simply open-circuited by the fuse. One of the major problems in multi-series group capacitor banks is the limitation imposed on designs because of a voltage doubling effect that occurs when a capacitor unit fails and its fuse clears near the first current zero. The other capacitors in the same series group as the failed unit can within a brief time, approximately half a cycle, after the fuse clears, sustain a large trapped voltage. This high voltage on those units imposes high energy absorption and clearing voltage requirements on the fuse. Efforts to avoid this problem tend to go in the direction of higher cost for fuses and capacitor units. In addition, as described above, in series capacitor applications, this doubling effect requires that there be provided one set of over-voltage protection equipment, such as a spark gap and a reactor, for each series group of capacitors.
What the present invention does, in brief, is to permit use of a capacitor bank of multi-series groups in which the capacitor units and their associated fuses may be of conventional design, and furthermore, do not require an individual set of major bypass equipment for each series group of capacitors. These purposes are achieved by an arrangement that includes a voltage limiter, or surge suppressor, assembly on each series group where the voltage limiter is characterized by being self-extinguishing, i.e., not a conventional spark gap or silicon carbide device. Of available components, the most suitable for high voltage equipment to which the present invention is directed are metal oxide voltage limiters such as those principally comprising zinc oxide. However, it is also significant to the practice of this invention that, in contrast to other proposed applications of metal oxide voltage limiters in capacitor protection equipment, the invention comprises a combination in which these voltage limiters are set at a protection level that is greater than that of the bypass equipment across the bank or any involved portion of it. For example, using terminology used in the trade, the capacitor units themselves may have a design capability of sustaining transients below 3.0 per unit (P.U.). That is, no protection or bypass equipment is required to operate so long as the voltage to which the capacitor bank is subjected is less than three times the nominal capacitor voltage. In this case, a set of bypass equipment is provided to operate at the 3.0 P.U. level. This bypass equipment may be a conventional spark gap and reactor configuration in accordance with known practice but which, in contrast with most prior practice, can now be applied across a plurality of series groups of capacitors. The additional voltage limiters provided across each series group in accordance with the invention are set at a higher protective level such as about 3.5 P.U. so their operation does not occur normally upon a line transient which would be taken care of by the protective equipment. These voltage limiters come into play upon the peculiar occurrence of the voltage doubling effect produced during fuse clearing of a failed unit. This may occur, for example, if the fuse of a defective capacitor unit operates when the voltage is at 2.9 P.U. Then there could be a rapid build-up of voltage across adjacent units within the next half cycle up to about 5.8 P.U. But even at this high level, the voltage across the bank seen by the protective equipment may still be below the 3.0 protection level. The individual arrangement of the voltage limiters in accordance with this invention across each series group permits their operation during the fuse clearing voltage doubling effect to briefly bypass just that series group and, because of the self-extinguishing character of such voltage limiters, to permit reinsertion of that series group within a very brief time.
As referred to above, one of the particularly favorable aspects of this invention is the opportunity to reduce the number of major bypass units necessary for the capacitor bank, as it is no longer necessary to provide such bypass equipment across each series group. There is, of course, a further trade-off that one encounters. As the spark gap or other bypass device is provided across larger numbers of series groups, there is need for it to have greater capability resulting in greater size and cost. It is considered that this trade-off will still result in over-all advantages of cost and performance at least to the extent of providing bypass equipment only across each half of a bank or each two series groups, but it is believed that the invention may be applied up to the extreme case in which the bypass equipment is across the entire bank. What is most evident is that the invention affords much greater design flexibility in determining the number and nature of the main power bypass equipment as such equipment no longer has to be designed to take into account the voltage doubling effect on fuse clearing of a failed capacitor unit.
While the present invention is particularly advantageous in applications such as series capacitor banks and transmission lines, it is not so limited. In a typical shunt bank application of capacitors, there are also employed multi-series groups of capacitors, with fuses, that are susceptible to the voltage doubling effect on fuse clearing. Here too, voltage limiters in accordance with the invention may be applied across each series group to take care of the voltage doubling problem. In common with the application in series transmission lines, these units would have a design protection level higher than that which otherwise operates in the system. That is, if there is bypass equipment, an arrestor, connected across the bank, these units would have a design protective level higher than that for operation of that bypass equipment. Even if there is not bypass equipment across the entire bank, these voltage limiters would be set at a protection level that is higher than that of the design capability of the individual capacitor units so that they don't enter into operation or influence anything unless the peculiar instance of the voltage doubling on fuse clearing occurs.